This invention relates generally to electronic timing circuitry and is particularly directed to accurately controlling the occurrence of an output signal from a timing circuit generated in response to an input signal provided thereto.
The timing of various signals in an electronic circuit is generally critical for its proper operation, particularly when used in digital applications. For example, the timing between a first input signal and a second output signal is frequently of the utmost importance with regard to the proper operation of the system in which the two signals are utilized. Defining and maintaining proper timed relation between the two signals is frequently referred to as "synchronizing" the two signals which may ideally occur substantially simultaneously or with a predetermined, well-defined interval therebetween.
One area in which signal synchronization is of critical importance is in the processing and display of video information on a raster scanned cathode ray tube (CRT) in a conventional television receiver. One approach to signal sychronization in a television receiver utilizes a phase locked loop in which a phase detector has horizontal rate synchronizing pulses applied to one input and ramp signals related to horizontal deflection within the CRT applied to the other input. The phase detector responds to these inputs and produces control pulses which are filtered and used to control a horizontal-rate oscillator at a frequency which is an average of the incoming synchronizing signals. The horizontal oscillator in turn drives a horizontal deflection generator producing recurrent trace pulses which are integrated to produce the recurrent ramp or sawtooth signal input to the phase detector. Operating limitations in a phase locked loop system arise, however, when input synchronization signals vary over a large frequency range. This variation of input synchronization signal frequency becomes a critical operating parameter when a video display is used in a non-television type of application. For example, a video display used in a computer terminal or in a data display system may be required to interface with a great variety of input synchronization signals.
An improved approach to the synchronization of horizontal sweep in a video display with video synchronization input pulses which avoids the limitations of a phase locked loop is the subject of U.S. Pat. No. 4,253,117 issued in the name of the present inventor. This approach makes use of a free-running astable multivibrator in the horizontal drive circuit of the video display in combination with a monostable multivibrator responsive to a horizontal drive or synchronization input signal for triggering the astable multivibrator in well-defined time relation with respect to the horizontal drive signal. Upon receipt of a synchronization signal the monostable multivibrator is driven to an unstable state in which it remains for a predetermined time. A reduction in synchronization input signal amplitude due to an input signal voltage cutoff threshold which reduces spurious multivibrator inputs is compensated for by coupling the synchronization input from the monostable multivibrator to the emitter and collector of both of the astable multivibrator's coupled transistors. This multivibrator combination, by increasing synchronization signal injection to the astable multivibrator, provides an increase in the synchronization signal frequency capture range of more than three times that previously available.
However, this approach, as well as other prior art multivibrator synchronization signal arrangements, is not without limitations. For example, prior art systems typically vary the delay between output signal generation and the receipt of the input signal by changing the RC time constant of a timing circuit in the multivibrator such as by including a potentiometer therein. This results in a change in the discharge rate of the monostable multivibrator's RC timing circuit and introduces unwanted variation in the triggering of the following stage. In addition, changing the multivibrator's time constant permits only limited variation of the circuit's duty cycle and thus limits the timing range of the output signal relative to the received input signal.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a monostable multivibrator which includes an RC timing circuit having a uniform time constant over a range of time delays and wherein the duty cycle may be varied over a range of from approximately 1% to 90% by changing the value of a single component therein.